Recently, more and more researches are focus on forming a Group III-nitride based light emitting device (LED) structure on a Group IV-based semiconductor substrate. This is because the Group IV-based semiconductor substrate is much cheaper than that of the typical LED substrates, such as sapphire substrate or the silicon carbide (SiC) substrate. The LED structure formed on the Group IV-based semiconductor substrate is also much easier to be integrated on the integration circuits formed on the Group IV-based semiconductor substrate, or could be easily compatible with the fabrication process of the integration circuits on the Group IV-based semiconductor substrate. Nevertheless, the hetero-junction structure existing between the Group III-nitride based LED structure and the Group IV-based substrate usually brings some structural problems due to the mismatch of two different crystalline lattices and/or the difference of coefficient of thermal expansion (CTE) between these two materials in the hetero-junction structure. The mismatch of the hetero-junction structure always causes bad epitaxy quality of LED structure, which might greatly affect the optical property of the light emitting device.
There are many references relating to the fabrication of the Group III-nitride based LED structure on the Group IV-based substrate. C. A. Tran disclosed in Applied Physics Letters (1999) a method for growing an InGaN/GaN multiple quantum well (MQW) blue light emitting diodes (435 nm) on a silicon (111) substrate by the metalorganic vapor phase epitaxiy (MOVPE) process, where the LED is operable in 4 volts. However, the epitaxial film of such LED would be cracked due to the stress existing between the epitaxial film and the silicon substrate.
B. J. Zhang et al. also disclosed in Phys. Stat. Sol. (a) (2001) an InGaN multiple quantum well (MQW) light emitting diodes (LED) structure formed on a silicon (111) substrate. The MQW LED is fabricated by the steps of forming an n-type AlN/AlGaN (120/380 nm) buffer layer in the temperature of 1130° C. by the MOCVD, forming an n-type GaN layer of 0.2 μm, forming an In0.13Ga0.87N quantum well of 3 nm, forming an In0.01Ga0.99N barrier layer of 5 nm, forming a p-doped layer of Al0.15Ga0.85N of 20 nm and forming a p-type GaN cover layer of 0.2 μm. Although the crack does not occur in the LED structure disclosed by B. J. Zhang et al., it is clear that the formation of the n-type AlN/AlGaN buffer layer in the temperature of 1130° C. could likely result in the formation of the AlSi alloy since the eutectic point thereof is about 577° C. Thus, the epitaxy quality of the LED structure could be affected by the formation of the AlSi alloy.
A. Dadgar et al. also disclosed in Phys. Stat. Sol. (a) (2001) a crack free InGaN/GaN LED structure on a silicon (111) substrate. Such LED structure is fabricated by the steps of forming a patterned silicon nitride on a silicon substrate by a sputtering process, and then forming a silicon-doped AlN layer, 15 pairs of AlGaN/GaN multiplayer structure, GaN:Si structure of 300 nm and three-layered InGaN/GaN quantum well on the pre-deposited aluminum layer. Although such LED structure is crack free, the formation of the patterned silicon nitride will occupy many areas of the silicon substrate, which results in the decrease of the effective area of the LED.
In addition, please refer to FIG. 1(A) and FIG. 1(B), which respective show a conventional nitride LED structure formed on a SiC/Si substrate and on a SOI (silicon on insulator) substrate according to the U.S. Pat. No. 5,786,606 by Johji Nishio et al. The conventional LED structure is mainly focused on forming a silicon layer on a Si or SiC substrate having thereon a silicon oxide (SiO2) layer, and then forming the LED active layer on the silicon layer. After the formation of the LED active layer, the silicon oxide layer is removed by a wet etching process, so as to form the LED structure shown in the respective FIG. 1(A) and FIG. 1(B). Nevertheless, it is clear that the fabrication processes for the LED structures in the respective FIG. 1(A) and FIG. 1(B) are much complicated, time consuming and costly.
In order to overcome the above-mentioned issues, a novel light emitting device (LED) structure on a Group IV-based semiconductor substrate and the fabrication method therefore are provided. In such a light emitting device (LED) structure and the fabrication method, the epitaxy quality and the optical property of the LED structure on a Group IV-based semiconductor substrate will be greatly improved.